1. Field of the Invention
The present invention relates to a multi-stage process for isomerizing olefins. This invention is particularly well-suited for isomerizing an alkenyl bridged ring compound to the corresponding alkylidene bridged ring compound. The advantages of the multi-stage process are provided by use of a highly active and highly selective isomerization catalyst. The process of the invention is particularly useful for isomerizing 5-vinyl-2-norbornene (hereinafter "VNB") to 5-ethylidene-2-norbornene (hereinafter "ENB"), which is used commercially in the production of elastomeric polymers and synthetic rubber. The highly active isomerization catalyst is prepared by adding an alkali metal to alumina and then activating the mixture by careful oxidation of the alkali metal.
2. Description of the Prior Art
Isomerization of olefins is well-known in the art. An incentive for isomerization arises when the olefinic double bond in the starting isomerizable olefin must be moved to a different position in order to provide a properly reactive olefin. For instance, it is known to isomerize pentene-1 to pentene-2 so that pentene-2 may be used in the catalytic alkylation of an isoparaffin. The resulting alkylate is useful as an additive to enhance octane in gasoline. Also, the double bond in an olefinic intermediate compound may need to be shifted in position in order to continue synthesis of the desired final chemical product.
Isomerization is also used to provide olefins necessary for polymerization. One such olefin is ENB. The use of ENB as a monomer in the production of rubbery polymers is well known. ENB may be produced by reacting 1,3-butadiene and cyclopentadiene in an addition reaction commonly known as a Diels-Alder reaction, yielding VNB which is then catalytically isomerized to ENB.
Known isomerization catalysts include liquid bases, such as mixtures of alkali metal hydroxides and aprotic organic solvents, mixtures of alkali metal amides and amines, and mixtures of organic alkali metal compounds and aliphatic amines. Unfortunately, the catalytic activity of the liquid bases is relatively low, and therefore a large amount of these expensive catalysts must be used. Also, recovery of the catalyst from the reaction mixture is very difficult, requires complicated separation and recovery steps and consumes a large amount of energy.
Solid isomerization catalysts are also known, for example, alkali metals carried on large surface area anhydrous supports such as activated carbon, silica gel, alumina and the like. These solid catalysts are difficult to handle because they may ignite and lose activity on contact with oxygen. Also, the isomerization performance of these solid catalysts is poor, because conversion and selectivity are low.
U.S. Pat. No. 3,897,509 discloses that heating an alkali metal, an alkali metal hydroxide and alumina yields an alkali catalyst composition which is described to be stable on exposure to air and water and active in various chemical reactions. Particularly the catalyst is said to be useful in the isomerization of alkenyl bridged ring compounds to the corresponding alkylidene bridged ring compounds. Thus, it is useful for the production of alkylidene bridged ring compounds such as ENB which is valuable in the production of synthetic rubber.
The preferred method of forming the catalyst of U.S. Pat. No. 3,897,509 is by heating and mixing an alkali metal, an alkali metal hydroxide and alumina at a temperature higher than the melting point of the alkali metal; however, the catalyst may also be prepared without the use of an alkali metal hydroxide if the starting alumina contains water.
U.S. Pat. No. 3,405,196 discloses a process in which a terminal olefin is converted to an internal olefin in the presence of a supported alkali-metal catalyst that has been pretreated with an oxygen containing gas such as nitrous oxide. The catalyst used contains an alkali metal dispersed on a high-surface area, substantially inert support. The alkali metal may be selected from sodium, potassium, rubidium and cesium. The desired catalyst support material is a high surface area, large pore, and slightly acidic alumina. The catalyst is primarily used to convert 1-pentene to 2-pentene and 1-butene to 2-butene and is not used to isomerize an alkenyl bridged ring compound to an alkylidene bridged ring compound.
Many of these prior art catalysts will react violently with water and suffer from poor conversion activity. The catalysts also deactivate on contact with catalyst poisons. These and other shortcomings of the prior art are overcome by the present invention, and a new multi-stage process for isomerizing olefins is provided.